This is probably going to be very badly done, I'm not a stats guy, so if I'm wrong, please understand, you're my first victim
Looking at Carrick's numbers from Hockey-Reference, if I understand these things correctly, Carrick is:
1) A positive in both Corsi F% and %rel for the past two years. A good thing.
2) Fenwick and PDO for the past two years are also positives and again these are good things.
3) His zone starts heavily favor the O vs the D. 60/40
My interpretation of these numbers and what I see from Carrick as a 3rd pairing D, one who is sheltered, is that he's not dragging the team down into negative territories, which would indicate the Leafs were being out-chanced when he's on the ice. As a third pairing Dman (for the most part) I'm ok with that type of result. Every team would like to improve the 5/6/7 etc Dmen. So would I. However, as long as they are not leading to play occurring in your zone more than in the O or Neutral zones, then I think they can be seen as doing their job. Carrick has been better than Polak (who Imo, has picked up his play the past 10 days odd) and isn't killing the Leafs in the 3rd pairing and as his game totals grow, I see room for improvement. Oh, and off the top of my head, I'm guessing that with his high O zone start rate he might often be on the ice with JVR and Bozak as they too have high O zone start rates and they're hardly known as defensive stalwarts.
Hope this made sense, be kind